Flow meters are used to measure the mass flow rate, density, and other characteristics of flowing materials. The flowing material may comprise a liquid, gas, solids suspended in liquids or gas, or any combination thereof. Vibrating conduit sensors, such as Coriolis mass flow meters and vibrating densitometers typically operate by detecting motion of a vibrating conduit that contains a flowing material. Properties associated with the material in the conduit, such as mass flow, density and the like, can be determined by processing measurement signals received from motion transducers associated with the conduit. The vibration modes of the vibrating material-filled system generally are affected by the combined mass, stiffness, and damping characteristics of the containing conduit and the material contained therein.
A typical Coriolis mass flow meter includes one or more conduits that are connected inline in a pipeline or other transport system and convey material, e.g., fluids, slurries and the like, in the system. Each conduit may be viewed as having a set of natural vibration modes, including for example, simple bending, torsional, radial, and coupled modes. In a typical Coriolis mass flow measurement application, a conduit is excited in one or more vibration modes as a material flows through the conduit, and motion of the conduit is measured at points spaced along the conduit. Excitation is typically provided by an actuator, e.g., an electromechanical device, such as a voice coil-type driver, that perturbs the conduit in a periodic fashion. Mass flow rate may be determined by measuring time delay or phase differences between motions at the transducer locations. Density of the flow material can be determined from a frequency of a vibrational response of the flow meter. Two such transducers (or pick-off sensors) are typically employed in order to measure a vibrational response of the flow conduit or conduits and are typically located at positions upstream and downstream of the actuator. The two pick-off sensors are generally connected to electronic instrumentation by cabling, such as by two independent pairs of wires. The instrumentation receives signals from the two pick-off sensors and processes the signals in order to derive flow measurements.
One potential source for error in vibrating flow meters is caused by compressibility, also known as velocity of sound effects. These errors generally increase with increasing tube oscillation frequency and therefore, the errors often occur during high frequency operation. A number of models have been developed to characterize the velocity of sound effects in a vibrating flow meter. For example, the error effects in both the measured density and mass flow rate were characterized by Hemp J and Kutin J., Theory of errors in Coriolis flowmeter readings due to compressibility of the fluid being metered. Flow Measurement and Instrumentation, 17:359-369 (2006), as:
                              ρ                      vos            ,            err                          =                              1            4                    ⁢                                    (                                                ω                  ⁢                                                                          ⁢                  d                                                  2                  ⁢                                                                          ⁢                  c                                            )                        2                    ×          100                                    (        1        )                                                      m            .                                vos            ,                                                  ⁢            err                          =                              1            2                    ⁢                                    (                                                ω                  ⁢                                                                          ⁢                  d                                                  2                  ⁢                                                                          ⁢                  c                                            )                        2                    ×          100                                    (        2        )            
where:                ω=the angular oscillation frequency        d=the inner diameter of the flow tube        c=velocity of sound of the process fluid        
Therefore, if the velocity of sound in the process fluid is known, the error in the measured density and mass flow rate can be determined and corrected. Prior art solutions have generally addressed the situation where the process fluid comprises a mixture having two or more phases where the velocity of sound of the individual phases is known. For example, PCT patent application PCT/US07/74711, assigned to the present applicant, which is incorporated herein by reference, discloses a method for determining a velocity of sound for a multiphase flow mixture based on known velocity of sounds for the components. It should be understood that the equations listed above as well as the equations provided in the above referenced PCT patent application are merely examples of a model for VOS effects on a vibrating tube. Other models are known and are within the scope of the description and claims. The specific example given above and the examples used throughout should not limit the scope of the present invention.
In many circumstances, for example, if a gaseous mixture has an unknown composition, the velocity of sound may not be known. Furthermore, even if the composition is known, the velocity of sound for those components may be unknown. Other prior art solutions have employed additional sensors, such as acoustic sensors to measure the velocity of sound. This approach is not only more costly, but may be impractical in many situations due to space and cost restrictions.
Therefore, there is a need in the art for a method of obtaining a velocity of sound value based solely on measurements obtained from a vibrating meter. Furthermore, there is a need in the art for obtaining a velocity of sound measurement of a single phase fluid where the components are unknown. The present invention solves this and other problems and an advance in the art is achieved.